This invention relates to magnetic tape recording and reproducing systems and more particularly to a system in which tape marker sensors produce output signals representing the beginning and the end of the tape.
Magnetic tape recording and reproducing units are extensively used with digital data processing systems. Examples of such systems include the STC-3400 and 3600 tape drives manufactured by Storage Technology Corporation, Louisville, Colo.
In such systems, the magnetic tape has reflective markers which mark the beginning of the tape and the end of the tape. Photo-detectors are positioned to detect these markers. When the tape passes the sensor, a signal representing the position of the tape is applied to the tape control system.
In U.S. Pat. No. 3,614,020 Herger, an additional sensor is positioned adjacent the vacuum column on the machine reel side of the magnetic heads. When the end of the tape marker passes this sensor, the machine is switched from a high-speed rewind to a low-speed rewind. This provides a time saving in the rewind operation.
In these systems, it is important to reliably distinguish the change in reflectivity when the reflective marker passes the sensor. Changes in the intensity of the light source with time, thermal drift, and variable tape reflectivity complicate this task. In the prior art systems, a potentiometer is provided to control the voltage level of the photo-detector. Periodically, the service engineer checks the operation of the system and adjusts the potentiometer so that the voltage level is reasonably constant to allow reliable detection. Changes in the reflectivity of the tape present a more serious problem. The reflectivities of the markers are reasonably consistent. However, the reflectivity of the tape varies markedly among the tapes of different manufacturers. Because of this, the service engineer often has to make an adjustment in the potentiometer setting when a tape reel from a different manufacturer is placed on the machine. This is disruptive of good performance of the magnetic tape system.